This invention relates generally to methods of restoring contaminated soils and groundwater in-situ using calcium chloride, and, more particularly, to a method for stabilizing soluble heavy metal contaminants in carbonate-containing land or submarine formations using aqueous solutions of calcium chloride to prevent the remobilization of these heavy metal contaminants by groundwater.
There is increasing concern over the hazards posed by the rising levels of heavy metals within the world's water supplies and land formations due to accidental spills, leaks, mining practices and poor disposal practices. Most heavy metals are toxic to some degree to all life-forms, and can have a deleterious effect on aquatic flora and fauna. In humans, toxic heavy metal poisoning can lead to severe nervous system disorders and can cause death.
Accordingly, numerous methods for the removal of heavy metals from land and submarine formations have been proposed. For example, contaminated water or soil could be removed from its location and subsequently treated offsite. However, this method is not only extremely time-consuming and costly, it presents added dangers associated with the transportation of such hazardous materials. In addition, it may not be feasible or practical from a cost standpoint to remove an entire contaminated area for treatment. Thus, significant amounts of contamination may remain even after application of such treatment methods.
Alternatively, it has been suggested that heavy metals situated in ground-water or soil can be mobilized and removed via solubilization techniques, wherein the effluent is subsequently pumped above ground, removed and treated. Again, such methods are typically very time-consuming and costly. In addition, this approach can potentially lead to undesirable further spread of the contamination.
It has further been suggested that heavy-metal-containing-soils can be treated in-situ by various precipitation techniques. For example, in U.S. Pat. No. 4,354,942, issued to Kaczur et al., a method for the in-situ removal of soluble mercury from land and water areas using sulfur compounds is described. However, the use of sulfur compounds presents potential safety and health problems in itself.
The use of various calcium-containing-compositions to treat heavy metals in ground-water and/or soil is also known. However, there are significant disadvantages associated with the application of each of these methods. For example, in U.S. Pat. No. 4,950,409, issued to Stanforth, there is disclosed a method for treating hazardous waste-containing soil using calcium carbonate in 35 the form of limestone. In Stanforth's method the soil must be intimately mixed with the limestone in order to render the soil nonhazardous. Accordingly, Stanforth's method is not intended to be utilized in-situ, especially underground, where such mixing would hot be feasible. Moreover, the addition of limestone underground would cause premature plugging once the solution was added, thereby preventing the entire contaminated area from being treated.
In U.S. Pat. No. 4,981,394, issued to McLaren et al., a method of inducing carbonate cementation or precipitation of calcite as a sealant in soils, sediment, bedrock, and other alkaline materials is disclosed. In this method, solid particles of calcium chloride with limestone fines are plowed into the surface of the solid waste landfill, which is then wetted down with solution. These components react to produce crystals of calcium carbonate which cement soil particles together and fill void space to reduce porosity and permeability. This method results in severe plugging of the flow path for the area being treated, and thus, prevents the entire contaminated area from being treated. In addition, this method merely prevents further migration of the contaminants instead of achieving the more desirable stabilization of them.
In U.S. Pat. No. 4,418,961, issued to Strom et al., there is disclosed an in-situ method for restoring soluble heavy metal contaminants in subterranean formations that have been subjected to uranium mining. Strom et al. teach to add restoration fluid containing small amounts of CO.sub.2 to a mining site containing calcium carbonate in order to induce precipitation of calcium with the heavy metal ions, and thereby stabilize the heavy metal contaminants. Alternatively, Strom et al. disclose that if the site does not contain calcium carbonate therein, or if the reaction with CO.sub.2 is not sufficient to reduce contaminants to acceptable levels, restoration may be achieved by injecting an aqueous solution of calcium chloride.
Strom's method is limited to removal of metal anions. Moreover, this method essentially limits the amount of calcium ion to be added to concentrations dictated by the solubility product of the calcium/heavy metal precipitate, which is a relatively minor amount. Accordingly, the method is slow, inefficient, and highly ineffective in reducing the level of contaminates to environmentally acceptable levels. As a result, groundwater in formations treated with Strom's restoration methods is likely to be re-contaminated with unacceptably high levels of heavy metal contaminants several weeks after treatment.